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Top end power loss around 4000 rpm - any suggestions?

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Top end power loss around 4000 rpm - any suggestions?

PostAuthor: johndavid400 » Sun Sep 13, 2020 9:37 pm

Hey guys,

I have a 95' xv750 with around 7k miles. My dad bought it in 96' with 1k miles on it from the original owner and I drove it every now and then in college from 2000-2005 - it ran like a dream with plenty of power. Dad is getting older and stopped riding, so it sat in a storage unit for several years before he gave it to me. When I got it in 2018, it would barely start when choked and could barely keep it running with full throttle. As you can imagine, the carbs were filthy.

I rebuilt both carbs (with a rebuild kit from ebay) trying not to touch any of the linkages between the 2 carbs to (hopefully) keep them in sync. I replaced both spark plugs, changed oil/filter, changed rear axle/differential oil, new tires, new air filter, new battery, lights, handlebar, etc. After the tune-up, it will crank almost immediately (no choke needed) and with an initial idler adjustment - it purrs and sounds like it did years ago. Idle is smooth, I can rev it up to high RPM with no issues (in neutral), no backfiring, and has plenty of low-end power (on par with how I remember when it was new).

The problem is, when I am on the highway and get it up to around the 4000 RPM range, it starts to lose power. If I shift into a higher gear and the RPM's drop, I get another surge of power until I get it back up to that 4000 RPM mark again. This means that my top speed is limited to about 60-65 mph in 5th gear before it starts losing power and hesitating. It hasn't actually been that big of an issue for riding, as the roads I ride on tend to be around 55mph speed limit, but since this was never a problem before it sat, I'm wondering if anyone has any suggestions as to what I might need to look at to fix it?

I initially thought the carbs need to be synced and I am planning on doing that soon (just in case), but also the symptoms of out-of-sync carbs seem to be rough idle and some other things that I have not experienced. To re-iterate, it cranks, idles, and runs perfectly until I am on the highway and get up to 4000 rpm, then it seems to lose power. Anyone else have this issue and have any advice?

Thanks in advance!

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Re: Top end power loss around 4000 rpm - any suggestions?

PostAuthor: Flyingdog » Mon Sep 14, 2020 1:18 am

:VTF:

At this point...I would say there's a vacuum leak some place. Pressures change from idle to being under loads. First place I'd check is the manifold-to-head seal. I've never trusted just those seals. Don't take much to lose the seal. Why "I" always use a smear of exhaust gasket maker, even on "new" areas.

johndavid400 wrote:I initially thought the carbs need to be synced and I am planning on doing that soon (just in case), but also the symptoms of out-of-sync carbs seem to be rough idle and some other things that I have not experienced.


Idles' fine so sync is fine? Not necessarily, regardless what others say, carbs "can" go outta sync at the high rpms. Can happen cuz of vacuum leaks (air pressures change with throttle positions), sloppy linkages, weak diaphragms, needle jets, those tubes with the black bulbs, etc etc etc. Why I suggest (atleast checking) sync at the higher rpms, does "not" always mean having to adjust.
Seemings how it screws up at a particular rpm, points to the manifolds. Are they cracked? Might look ok, but give them a squeeze and those cracks open up like the Grand Canyon. Same thing happens "under load", when the pressures change with throttle position.
Don't forget to check all that EPA stuff/including the hoses (specially at the nipples. Little cracks can't always be seen.) Good sources for leaks and/or MCV not working.
Hey, it's a start. Something "stored" develops all kinds of weirdness, as you've discovered. :bg: :cl:
Last edited by Flyingdog on Mon Sep 14, 2020 1:55 am, edited 2 times in total.
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Re: Top end power loss around 4000 rpm - any suggestions?

PostAuthor: faffi » Mon Sep 14, 2020 1:48 am

Welcome!

There are 2 typical reasons for the issues you describe: lack of correct air/fuel mixture and retarded ignition timing. Whereas the latter is possible, it is not very likely due to everything being handled electronically. However, if the trouble seems to be rpm related and not load related - meaning the issue will always appear at the same rpm, regardless of you using little or full throttle, ignition timing is well worth checking with a strobe timing light.

Usually, though, with the bike's history, I would guess the carbs suffer from internal blockage. A repair kit does not always fix this; there are many passages inside that can be blocked without you being able to see it.
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Re: Top end power loss around 4000 rpm - any suggestions?

PostAuthor: Flyingdog » Mon Sep 14, 2020 2:17 am

johndavid400 wrote:The problem is, when I am on the highway and get it up to around the 4000 RPM range, it starts to lose power. If I shift into a higher gear and the RPM's drop, I get another surge of power until I get it back up to that 4000 RPM mark again. This means that my top speed is limited to about 60-65 mph in 5th gear before it starts losing power and hesitating.


faffi wrote:trouble seems to be rpm related and not load related


Hmm...sounds "load related" to me. Seemings how everything seems fine when revving in neutral.
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Re: Top end power loss around 4000 rpm - any suggestions?

PostAuthor: Hellgate » Mon Sep 14, 2020 2:47 am

Sounds like the fuel filter is clogged. It can deliver enough fuel at lower rpms but too clogged to run well at higher rpms.
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Re: Top end power loss around 4000 rpm - any suggestions?

PostAuthor: Flyingdog » Mon Sep 14, 2020 4:03 am

:yup:

Good point..and/or fuel level in float bowls. Something not keep'n up with the demand.... Pretty much alotta process of eliminations. Tweak this/go for a ride..tweak that/go for a ride...good ole fashion detective work...and...pita. :bg:
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Re: Top end power loss around 4000 rpm - any suggestions?

PostAuthor: johndavid400 » Tue Sep 15, 2020 3:35 pm

Thanks for the replies and suggestions.

I will check the rubber boots going from the carbs to the head.. they appeared to be in good shape when I re-installed them, but could have missed something. Is there a way to check this without taking the carbs off (as you know that is not fun)? I vaguely remember someone saying you could spray something on them while running to see if the RPM's suddenly change (can't remember what they said to spray..)

I did replace a few vacuum lines during the tune-up, but there is a baffle on the left side just in front of the carbs that has several vacuum lines and there could be a leak in one of those. Does anyone know of a checklist to go through or have suggestions on how to troubleshoot vacuum lines leaking?

Still planning on syncing the carbs.. do you guys recommend buying a gauge kit or doing the DIY tubing with transmission fluid method? I read mixed things about the needle style gauges and was hoping to not have to spend $100 on a motion pro unit.

I suspected the fuel filter - so I took off the tank and pulled the petcock off and it was clean as a whistle. I took it off anyways and cleaned it with my air compressor, but I ruled that out pretty quickly. Also carbs are getting plenty of fuel.. I tested by putting a clear tube on the bottom of each and opening the bowl drain screws... gas was freely coming out in a stream as thick as the tube, so pretty sure they are getting the fuel. Obviously there could be something clogged in the carb(s). I watched every youtube video I could find on cleaning/rebuilding them and got every spot people said to take care of... but could have missed something. Also, I set the bowl floats to 27mm (if I recall correctly) according to the manual and guys on youtube, and followed the method several people suggested on setting the float height - so I hope I did that right because getting the carbs off and back on was a pain (that one top bolt on the rear boot takes forever to remove/reinstall).

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Re: Top end power loss around 4000 rpm - any suggestions?

PostAuthor: Hellgate » Tue Sep 15, 2020 4:29 pm

I would have bet money on the fuel filter. I hope you figure it out without too much effort. I'm curious to learn what you discover.
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Re: Top end power loss around 4000 rpm - any suggestions?

PostAuthor: Jake » Tue Sep 15, 2020 5:15 pm

Under the needle seat is a fine screen. Remove the needle seat and check that, many times that little screen is clogged.
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Re: Top end power loss around 4000 rpm - any suggestions?

PostAuthor: Flyingdog » Tue Sep 15, 2020 6:20 pm

johndavid400 wrote:Still planning on syncing the carbs.. do you guys recommend buying a gauge kit or doing the DIY tubing with transmission fluid method?


IMO: I always use a dedicated vacuum gauge. These are the ones I used for yrs. Coarse I probly do afew more than you. :bg:
https://www.amazon.com/ALPHA-MOTO-Synch ... B004MSE9JO
They're copies of the K & L(s), but bout $100 cheaper. As you've notice, there's all shapes and sizes out there. All will do whatcha need. I would imagine even the DIY(ers) would work fine, for those looking for something to do. I just never mess with DIY stuff where I need the most accuracy.
Far as checking for vacuum leaks, I just use carb cleaner. Some use (unlit) propane or WD (messy). I only mentioned the manifold (0-ring) seal, cuz, sometimes (not often), that 0-ring can roll outta the groove while install'n them back on the heads.


Next time you have the carbs off..replace the bolts (manifold and float bowl) with SS allen bolts. If ya don't wanna replace, you can atleast ditch that funky collar washer.
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Re: Top end power loss around 4000 rpm - any suggestions?

PostAuthor: Flyingdog » Wed Sep 16, 2020 3:39 am

Hellgate wrote:I would have bet money on the fuel filter.


I had a buck against that. Didn't you get the memo?. :bg:

Personally...don't care what it is (car, truck, motorcycle), why "I" always change any OEM inline filter to a clear glass one. Can just look at em...no guessing. Level any higher, it's probly clogged.

Fuel Filter level..normal.JPG
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Re: Top end power loss around 4000 rpm - any suggestions?

PostAuthor: chrismalm » Thu Sep 17, 2020 6:13 pm

Ok, I'll play. Here's my :2cents: ,
Do a clear tube fuel level check while the bike is running. Try to hold it at 4k rpm and see if the fuel level goes down in either carb. That should help you determine if it's fuel starvation. Run a fan on it and don't let it overheat.

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